Astrid Arkham was the daughter of Jeremiah Arkham raised in Arkham Asylum surrounded by it's inmates. As a young woman she took up the alias of the Arkham Knight and founded the Order of the Sun, a cult dedicated to destroying Batman.
Astrid's super-villain persona originated in the video-game Batman: Arkham Knight although in it the knight had a different true-identity. She is however the first incarnation of the Arkham Knight in mainstream comics, first appearing in Detective Comics #1000
Astrid Arkham was the daughter of Dr. Jeremiah Arkham and Dr. Ingrid Karlsson, both of whom were psychiatrists at Arkham Asylum. Her mother Ingrid was a kind woman who was very well liked and respected among the inmates and staff alike, even among the likes of the Joker. Tragedy struck however when Ingrid was pregnant and went into labor during a riot among the inmates. Clayface, Poison Ivy, Joker, Harley Quinn, and Solomon Grundy protected Ingrid during the riot and aided her in delivering Astrid only for a stray batarang to be thrown by an inmate to slash Ingrid's neck.
As a child, Astrid was isolated from society by her father and spent the entirety of her childhood within the confines of Arkham Asylum. Here rather than making friends with other children, Astrid befriended the inmates of Arkham Asylum many of whom served as a sort of surrogate parent to her. From these inmates Astrid came to hate Batman who she saw as a demon who hurt the citizens of Gotham. As a child she also developed a liking for knights in the fairy-tales which she would read to the Joker.
Astrid became obsessed with destroying the Batman and obsessively trained herself to be a skilled fighter and sword-fighter. By the time that she was in her late-teens or early 20s she equipped herself with a militaristic arsenal and took up the alias of the Arkham Knight. She even went so far as to found a cult known as the Order of the Sun which was dedicated to removing Batman from Gotham.
Major Story Arcs
Future State: The Next Batman